Disclosed is a gas sensor including a metal shell, a ceramic holder placed in an axial inner hole of the metal shell and a sensor element inserted through an insertion hole of the ceramic holder. The ceramic holder has a recessed hole recessed toward the rear from a front-facing surface of the ceramic holder. The sensor element has, at a front end part thereof, a detection portion covered with a porous protection layer such that a rear end part of the protection layer is accommodated in the recessed hole with a space left therebetween. Further, the ceramic holder has a front circumferential edge defined between an inner circumferential surface of the recessed hole and the front-facing surface of the ceramic holder such that the whole of the front circumferential edge is located radially inside of a radially innermost position of the axial hole.